Doc/lib/librotor.tex

   1 \section{Built-in Module \module{rotor}}
   2 \label{module-rotor}
   3 \bimodindex{rotor}
   4
   5 This module implements a rotor-based encryption algorithm, contributed by
   6 Lance Ellinghouse\index{Ellinghouse, Lance}.  The design is derived
   7 from the Enigma device\indexii{Enigma}{device}, a machine
   8 used during World War II to encipher messages.  A rotor is simply a
   9 permutation.  For example, if the character `A' is the origin of the rotor,
  10 then a given rotor might map `A' to `L', `B' to `Z', `C' to `G', and so on.
  11 To encrypt, we choose several different rotors, and set the origins of the
  12 rotors to known positions; their initial position is the ciphering key.  To
  13 encipher a character, we permute the original character by the first rotor,
  14 and then apply the second rotor's permutation to the result. We continue
  15 until we've applied all the rotors; the resulting character is our
  16 ciphertext.  We then change the origin of the final rotor by one position,
  17 from `A' to `B'; if the final rotor has made a complete revolution, then we
  18 rotate the next-to-last rotor by one position, and apply the same procedure
  19 recursively.  In other words, after enciphering one character, we advance
  20 the rotors in the same fashion as a car's odometer. Decoding works in the
  21 same way, except we reverse the permutations and apply them in the opposite
  22 order.
  23 \indexii{Enigma}{cipher}
  24
  25 The available functions in this module are:
  26
  27 \begin{funcdesc}{newrotor}{key\optional{, numrotors}}
  28 Return a rotor object. \var{key} is a string containing the encryption key
  29 for the object; it can contain arbitrary binary data. The key will be used
  30 to randomly generate the rotor permutations and their initial positions.
  31 \var{numrotors} is the number of rotor permutations in the returned object;
  32 if it is omitted, a default value of 6 will be used.
  33 \end{funcdesc}
  34
  35 Rotor objects have the following methods:
  36
  37 \begin{methoddesc}[rotor]{setkey}{key}
  38 Sets the rotor's key to \var{key}.
  39 \end{methoddesc}
  40
  41 \begin{methoddesc}[rotor]{encrypt}{plaintext}
  42 Reset the rotor object to its initial state and encrypt \var{plaintext},
  43 returning a string containing the ciphertext.  The ciphertext is always the
  44 same length as the original plaintext.
  45 \end{methoddesc}
  46
  47 \begin{methoddesc}[rotor]{encryptmore}{plaintext}
  48 Encrypt \var{plaintext} without resetting the rotor object, and return a
  49 string containing the ciphertext.
  50 \end{methoddesc}
  51
  52 \begin{methoddesc}[rotor]{decrypt}{ciphertext}
  53 Reset the rotor object to its initial state and decrypt \var{ciphertext},
  54 returning a string containing the ciphertext.  The plaintext string will
  55 always be the same length as the ciphertext.
  56 \end{methoddesc}
  57
  58 \begin{methoddesc}[rotor]{decryptmore}{ciphertext}
  59 Decrypt \var{ciphertext} without resetting the rotor object, and return a
  60 string containing the ciphertext.
  61 \end{methoddesc}
  62
  63 An example usage:
  64 \begin{verbatim}
  65 >>> import rotor
  66 >>> rt = rotor.newrotor('key', 12)
  67 >>> rt.encrypt('bar')
  68 '\2534\363'
  69 >>> rt.encryptmore('bar')
  70 '\357\375$'
  71 >>> rt.encrypt('bar')
  72 '\2534\363'
  73 >>> rt.decrypt('\2534\363')
  74 'bar'
  75 >>> rt.decryptmore('\357\375$')
  76 'bar'
  77 >>> rt.decrypt('\357\375$')
  78 'l(\315'
  79 >>> del rt
  80 \end{verbatim}
  81
  82 The module's code is not an exact simulation of the original Enigma
  83 device; it implements the rotor encryption scheme differently from the
  84 original. The most important difference is that in the original
  85 Enigma, there were only 5 or 6 different rotors in existence, and they
  86 were applied twice to each character; the cipher key was the order in
  87 which they were placed in the machine.  The Python \module{rotor}
  88 module uses the supplied key to initialize a random number generator;
  89 the rotor permutations and their initial positions are then randomly
  90 generated.  The original device only enciphered the letters of the
  91 alphabet, while this module can handle any 8-bit binary data; it also
  92 produces binary output.  This module can also operate with an
  93 arbitrary number of rotors.
  94
  95 The original Enigma cipher was broken in 1944. % XXX: Is this right?
  96 The version implemented here is probably a good deal more difficult to crack
  97 (especially if you use many rotors), but it won't be impossible for
  98 a truly skilful and determined attacker to break the cipher.  So if you want
  99 to keep the NSA out of your files, this rotor cipher may well be unsafe, but
 100 for discouraging casual snooping through your files, it will probably be
 101 just fine, and may be somewhat safer than using the \UNIX{} \program{crypt}
 102 command.
 103 \index{NSA}
 104 \index{National Security Agency}